WASHINGTON, Aug. 7 (Xinhua) -- U.S. President Barack Obama said Thursday evening that he has authorized "targeted" airstrikes and airdrops of aid in Iraq to prevent a "genocide" by Islamist extremists.

Obama said in a speech that he authorized limited use of American air power against the Islamic State militants, if they advance on the city of Arbil in northern Iraq, where the United States has a diplomatic presence and advisors to Iraqi forces.

"When the lives of American citizens are at risk, we will take action," Obama said. "That's my responsibility as commander in chief."

"We can act, carefully and responsibly, to prevent a potential act of genocide," he said, referring to the attacks against as many as 40,000 Yazidi minority besieged on Mount Sinjar in northern Iraq following death threats from the militants.

The strike was also aimed "to help forces in Iraq as they fight to break the siege and protect the civilians trapped there," he added.

"When we face a situation like we do on that mountain, with innocent people facing the prospect of violence on a horrific scale, when we have a mandate to help -- in this case a request from the Iraqi government -- and when we have the unique capabilities to help avert a massacre, then I believe the United States of America cannot turn a blind eye," he said.

The president also announced that the United States launched airdrops of food and water to the civilians there.

The Pentagon has said a C-17 and two C-130 aircraft escorted by two F/A-18s had delivered thousands of gallons of drinking water and 8,000 packaged meals to the Yazidis.

The planes stayed over the drop area at a low altitude for 15 minutes, the Pentagon said.

The airstrikes would be the first carried out by the U.S. forces in Iraq since the end of 2011, but Obama vowed not to send back U.S. ground forces.

"As commander in chief, I will not allow the United States to be dragged into fighting another war in Iraq," he said, explaining "there is no American military solution to the larger crisis in Iraq."

Meanwhile, a U.S. official told reporters on condition of anonymity that the United States has not carried out any air strikes in Iraq but is ready to do so in Iraq anywhere it sees a threat to U.S. personnel or facilities.

"No air strikes have taken place at this time, but we remain postured to take targeted military action should the situation warrant it," the official said.

Earlier on Thursday, White House spokesman Josh Earnest told a press briefing that any U.S. military action would be "very limited in scope," and should be closely tied to Iraqi political reforms, which Washington has demanded.

As the U.S. built up its military forces in Iraq, the UN Security Council on Thursday condemned attacks by the Islamic State in the Middle Eadt country, demanding those responsible be held accountable.

"The members of the Security Council condemn" the attacks by the Islamic State and expressed its "deep outrage" at the displacement of hundreds of thousands of Iraqis, many of them from the vulnerable Iraqi communities, in the wake of the attacks, said Britain's UN Ambassador Mark Lyall Grant, who holds the rotating council presidency for August, after emergency closed-door consultations.

UN Secretary-General Ban Ki-moon called for international assistance to the affected population in northern Iraq.

"The secretary-general is deeply appalled at today's reports of attacks by the terrorist group Islamic State (IS) in Kirkuk, and Qaraqosh, and earlier attacks in Tal Afar and Sinjar district affecting mainly the vulnerable communities of Christians, Turkomen, and Yezidis," said a statement issued here by Ban's spokesperson.

Also on Thursday, the White House said that U.S. Vice President Joe Biden phoned Kurdistan President Massoud Barzani to discuss Kurdish forces' military action against the extremists and protection of Yezidi refugees.

"Vice President Biden relayed President Obama's offer of humanitarian assistance for the Yezidis and reaffirmed his commitment to take whatever actions necessary to protect Americans in Erbil, including targeted airstrikes," the White House said in a statement.